As plants consume the soil’s reserve of nutrients and other resources, they deplete the supply. Without restocking, the soil is bound to become weakened and unable to afford the necessary high-quality nourishment required for continued healthy plant growth. Therefore, every time you work the soil, it’s important to amend it with organic matter. A particular benefit to the gardener who grows fruits and vegetables (and eats them) is that foods grown in organically amended soil have no poisonous chemical residues.

In addition to replenishing nutrients, amendments improve the soil’s texture and drainage. A further benefit of organic amendments is that they break down within a season or two, adding to the soil’s fertility. Inorganic amendments such as gypsum or vermiculite, which break down at a much slower rate, do not affect soil fertility but can improve its texture immediately.

Commonly referred to as black or brown gold, compost is the substance that results from the partial decomposition of organic material. This valuable soil amendment can be dug in when preparing a bed, added to a planting hole or to an existing planting (as side-dressing in the top several inches of soil around the outer limits of the root zone), or even used as an organic mulch. Homemade compost, consisting of kitchen vegetable scraps, uncontaminated grass clippings, and garden debris (perhaps with the addition of well-rotted manure), allows you to recycle what might otherwise be waste or garbage into the best possible amendment for your soil. Easy to do, good for the environment, and great for the garden, composting inspires a sense of having done the right thing by returning to the soil what the plants have taken from it.

Compost provides the perfect amendment for any type of soil, improving aeration and drainage in heavy clay soil, increasing water retention and fertility in light, sandy soil, and maintaining good soil structure, tilth, and fertility in loam soils.

Temperature also plays a role in the decaying process. The microorganisms that break down the organic matter of the compost heap generate heat by releasing the energy locked in the debris. Called thermophilic (heat-loving) bacteria, these organisms remain at work as long as they have organic matter to feed on.

Turning the pile lets in the air that keeps the microorganisms active by giving them new materials to work on and sustains a high internal temperature to speed up the composting process. It also helps to aerate the pile, keeping leaves and other matter from matting and becoming odiferous. Generally, if a compost heap smells, it needs more air.

Compost and its hungry microbes need moisture as well as air. Ideally, the moisture content of the materials in the pile should be between 40% and 60%. During dry periods and in dry climates, it speeds things up considerably to sprinkle the pile with water at each turning or whenever a new layer of organic material is added. If the materials are already very wet (rain-soaked leaves, for example), it’s a good idea to mix in some dry organic material at the same time to maintain a good air-moisture balance. Diseased plant materials or seedheads (especially those of noxious weeds) should only be added to the compost pile with great caution. Most disease pathogens are killed when exposed to temperatures of 55°C for 25 minutes, and most weed seeds die at temperatures between 60°C and 65°C; however, it’s better not to invite the problem than to deal with the consequences of amending your soil with compost that contains active plant pathogens or viable weed seeds.

Methods

Early in the 20th century, Sir Albert Howard, assigned to improve conditions at a 300-acre farm at the Indore Institute of Plant Industry in India, discovered a relationship between what goes into the soil and what comes out of it. He noticed that many soil problems — poor tilth, waterlogging, compaction, surface crusting — led directly to the conditions that foster crop diseases and ultimately concluded that it is necessary to feed the soil rather than the plant. To this end, he developed the concept of multilayered compost to provide organic matter that would enrich the soil.

Since then, a great many theories have been advanced on how to produce the best compost. The only real difference among the methods, however, lies in the speed of decomposition, i.e., the length of time it takes the material to reach the compost stage.

Slow (Passive) Compost
This is the method for the gardener who is in no particular hurry for results. A slow compost pile, virtually nothing more than a pile of grass clippings and other garden debris and vegetable scraps from the kitchen allowed to decay in its own time, requires no work other than adding new material to the pile when at hand. Within several months, depending on the size of the pieces of compostable material, the pile will break down into black, crumbly, fertile compost.

Faster Compost
To speed up the process, the Indore method recommends layering the compost pile like a torte: a layer of leaves, weeds, and other vegetation alternating with a thin layer of manure, in a ratio of 3:1 (three times as much vegetation as manure). The volume of dry brown material (carbon) and green material (vegetative kitchen and yard waste) should be about equal. The addition of manure serves two purposes: It speeds up the rate of decay and — if well-rotted manure is used — is a virtually odorless method of recycling animal waste. If you’re lucky enough to have a fresh supply of manure (from a dairy farm, riding stable, or a friend who raises livestock), be sure not to add it to the compost pile until it’s completely broken down and indistinguishable from other vegetative matter; don’t ever apply it fresh. Well-rotted manures are also available at nurseries and garden centers.

Fastest Compost
To produce high-quality compost in the shortest period of time, maintain the center of the heap at a temperature between 40°C and 60°C. (Use a compost thermometer to check the temperature.) Don’t turn the pile as long as it remains between these two temperatures; instead, turn it when the temperature is either lower or higher. Ideally, the pile should be turned when the internal temperature reaches 60°C. This way, it neither gets so hot that it kills off the thermophilic bacteria nor reduces the heat to the point that the decomposition slows down. Obviously, this requires a bit of concentrated attention and work. The pile may need turning as often as every other day, but the payoff is finished compost in as little as three weeks.

There is much controversy among avid composters about turning vs. not turning (aerobic decay vs. anaerobic decay), large branches vs. small twigs. Ultimately these arguments are variations on the theme of how quickly the raw materials decompose into usable compost. Turned compost piles break down more rapidly than unturned compost piles, aerobic (oxygen-aided) decay is more rapid (and less odiferous) than anaerobic decay, and twigs or wood chips decompose faster than branches. The choice is between more work and less time or less work and more time: the compost will be essentially the same.

A practical way to compost in a small garden is to have two compost containers: one box can be filled, moistened, covered, and left to rot, while the other box is the active one that you fill with waste. By the time the second container is filled, the first should be completely rotted. If you have a lot of compostable material, you can use three bins placed side by side.

If you’re not inclined to build your own composter, many different kinds are for sale — from precut lumber for fashioning a slatted wooden bin to metal “tumblers” that are turned daily and can be used only if materials are shredded first. The middle road is the stackable composter. This method is best suited to households where the scale of the contemporary urban and suburban gardens precludes collecting great piles of leaves and letting them decompose in their own good time, and where large-scale wooden bins are equally inappropriate to the gardening space. Stackable composters can be used for the passive method, just to contain the pile, or for the rapid composting method. The three plastic tiers of the unit fit neatly on top of one another and are light enough to be handled easily. When the compost reaches the top of the unit, you simply fork material from the top onto the ground to form the bottom of a new bin. Fork the remaining material from the original stack to the new bin; restack the former middle and bottom sections. Use any completely composted material from the bottom of the original stack.